When laying pipelines on the seabed by means of a pipelay vessel where the pipeline is welded together from lengths of pipe, the pipe is normally laid in a dry state. This means that the pipe is full of air or another gas during laying.
According to known techniques of pipelaying, pipe stands are welded together to form a pipeline on the deck of the lay vessel. Alternatively, the pipe length is pre-welded and is wound onto a drum. For a so-called S-lay, the finished pipe is fed continuously over the side/stern of the ship and into the sea by means of an output ramp, a so-called stinger, which guides the pipe into the sea and ensures that the pipe does not experience too small a bending radius at the upper curve during the feeding out. Alternatively, the so-called J-lay method is used wherein the pipe is fed out vertically from the front of the vessel. Down at the seabed, the pipe assumes a new curve, the lower curve, where the bending radius is dependent on among other things depth, the weight of the pipe and the tensile force applied to the pipe by the lay vessel.
Thus during the laying, the pipe assumes the shape of an S or a J in the sea. When laying pipes at great depths, the mass of the section of the pipe located between the lay vessel and the seabed represents a considerable weight. By filling the pipe with gas, the buoyancy of the pipe helps relieve the lay vessel of the weight of the pipe. This makes it possible to use a lay vessel of a considerably smaller size than that which would be required if the pipe were to be full of liquid during the laying operation.
The greatest stresses experienced by the pipe during the laying operation are found in the bending zones immediately behind the lay vessel and down at the seabed. Instances are known where pipes have not been able to bear the stress to which they are subjected during laying, resulting in leakages. Such leakages may be due to defects in the welding or the material, or a stability problem may occur, which causes the pipe to collapse over a certain distance, creating 25 longitudinal cracks.
When a leakage occurs and the pipe fills with water, the pipe loses all or part of its buoyancy. Such leakages may have catastrophic consequences, in that the net weight of that portion of the pipe which is located in the sea between the lay vessel and the seabed exceeds the bed capacity of the lay vessel This may cause the pipe to detach from the production equipment on the lay vessel and fall down to the seabed, thus incurring further damage. It may also damage the vessel seriously. Obviously, such an unintended event or accident involves a great risk of personal injury and material damages. To secure against such an event one may use internal plugs within the pipeline to seal off the pipeline where an ingress of water occurred and prevent this water from filling the part of the pipeline suspended between the vessel and the seabed. Such plugs may be positioned close to a bend in the pipeline, close to the seabed or close to the vessel.
When water is penetrating into a gas filled pipeline, it is important to have a control system and plugs that can respond quickly and securely seal off the pipeline. Many known plugs will not react quickly enough and also possibly not give a satisfactory grip on the wall of the pipeline, and the plug may therefore slip in relation to the pipeline and therefore not seal off the pipe, which then gives an unsatisfactory security of the system. This issue has for several plug systems been solved by having two plugs connected to each other, and positioned on opposite ends of the area of the pipeline where a potential leakage may occur.
An object of the invention herein is to remedy the disadvantages of the prior art.
The object is achieved in accordance with the invention by the characteristics stated in the description below and in the appended claims.